1. Field of the Invention
The present invention is directed to a detector for sensing events in living tissue of the type having a comparator which compares an input signal corresponding to electrical signals from the tissue with a defined level of signal detection, an event being deemed to have occurred if the input signal exceeds the defined level of detection.
2. Description of the Prior Art
A pacemaker is disclosed in U.S. Pat. No. 4,041,953 which emits stimulation pulses to a heart when the heart does not beat spontaneously (naturally). In order to detect spontaneous events, the electrical signals of the heart are sensed, amplified and sent to a level detector which determines whether the amplitude of the cardiac signal exceeds a detection level. A spontaneous heartbeat is deemed to have occurred if the amplitude exceeds the detection level, and emission of a stimulation pulse is inhibited.
In U.S. Pat. No. 4,708,144 a pacemaker is disclosed which, in a similar manner, senses electrical signals from the heart and interprets the signal as representative of spontaneous cardiac activity if the amplitude of the signal exceeds a specified detection level. The detection level is automatically adjusted by measurement of the peak value of the R-wave in the cardiac signal, and a long-term average is calculated. The detection level is then set according to the average value determined in this manner.
A problem with amplitude detectors of this type is that the measurement signal often contains electrical noise from the surroundings. The physician must program the detector in the pacemaker disclosed in U.S. Pat. No. 4,041,953 with a specified detection level. This procedure is normally performed at a clinic in quiet, noise-free conditions. After the patient leaves the clinic, however, the detector may interpret noise from electrically noisier surroundings as spontaneous heart beats. The pacemaker would then be inhibited under circumstances when a stimulation pulse was actually necessary.
This problem is partially addressed by incorporating automatic setting of the detection level, as in the pacemaker disclosed in U.S. Pat. No. 4,708,144. The relationship between the detection level and the average value for peak values of R-waves results in some adaptation of the signal, picked up by the electrode, which contains the noise. The averaging of peak values takes place over a relatively long period of time, i.e., over at least several minutes. As a result, adaptation requires a few minutes after any change from a noise-free environment to a noisy environment. During this adaptation time, the noise may be interpreted as spontaneous heartbeats, or some spontaneous heartbeats could be missed. In addition, the noise must have a frequency different from the frequency of the spontaneous heartbeat rate if the level of detection is to be able to effectively compensate for that noise. This means that measurement of the R-wave will not help if the noise has the same frequency as the heart rate.
Another problem, which neither of the above-known two detectors addresses, is that of so-called far-field signals. Far-field signal is a signal which takes place in tissue which is relatively remote from the tissue being sensed, but which nonetheless is detected at the sensing site, and is thus interpreted as arising at the sensing site. For example, an event (spontaneous or stimulated) in the ventricle can be sensed in the atrium with an amplitude equal to the amplitude of a true event in the atrium. An ability to distinguish between signals with different sites of origins would thus be useful in many situations.